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Option Activity under Stock-Based Compensation Plans (Detail)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended
Mar. 31, 2019
USD ($)
$ / shares
shares
Shares Underlying Outstanding Options  
Beginning balance | shares 5,750
Options granted | shares 1,121
Options exercised | shares (7)
Ending balance | shares 6,772
Vested and expected to vest at March 31, 2019 | shares 6,500
Exercisable at March 31, 2019 | shares 3,673
Weighted-Average Exercise Price Per Share  
Beginning balance | $ / shares $ 18.20
Options granted | $ / shares 10.32
Options exercised | $ / shares 5.53
Ending balance | $ / shares 16.92
Vested and expected to vest at March 31, 2019 | $ / shares 17.08
Exercisable at March 31, 2019 | $ / shares $ 19.30
Weighted-Average Remaining Contractual Term (years)  
Balance at March 31, 2019 5 years 6 months
Vested and expected to vest at March 31, 2019 5 years 5 months 15 days
Exercisable at March 31, 2019 4 years 9 months 29 days
Aggregate Intrinsic Value  
Balance at March 31, 2019 | $ $ 230
Vested and expected to vest at March 31, 2019 | $ 229
Exercisable at March 31, 2019 | $ $ 186
Unvested  
Shares Underlying Outstanding Options  
Options cancelled | shares (73)
Weighted-Average Exercise Price Per Share  
Options cancelled | $ / shares $ 15.61
Vested  
Shares Underlying Outstanding Options  
Options cancelled | shares (19)
Weighted-Average Exercise Price Per Share  
Options cancelled | $ / shares $ 23.23